The present invention relates to an inverter which produces an output alternating voltage from a pulsating direct voltage for a load having an inductive component, particularly for the coil of an inductive cooking appliance. The frequency of the generated alternating voltage has an order of magnitude in the range of 10 to 100 kHz, particularly at about 30 kHz.
The output alternating voltage can be used to operate, for example, alternating current motors without commutation, inductors, transformers, fluorescent bulbs, at a higher frequency of about 30 kHz, and heating plates operating with inductive heating in which a magnetic field heats the container holding the food.
In the prior art, an inverter is known (see German Patent No. 3,400,671 or U.S. Pat. No. 4,614,998 ) in which a direct voltage is applied to a so-called half bridge including two series-connected power transistors. The transistors are alternatingly controlled to be conductive and generate the alternating voltage for the load at their point of connection. Between the successive conductive phases of the two transistors a rest period is provided as a safety measure in which both transistors are non-conductive. To permit the necessary current flow during these rest periods from the inductor load, free-running diodes polarized opposite to the collector-emitter paths of the transistors are connected in parallel with the transistors to temporarily take over the current from the load during the rest periods. A capacitor is connected in parallel with each diode to limit the steepness of the generated alternating voltage.
In such a prior art circuit it is also known to connect the connection point between the diodes, which are connected in parallel with the collector-emitter paths of the transistors, with the load, one the one hand, and, via a third winding, with the connecting point of the transistors, on the other hand. The currents from the two transistors flow alternatingly through this third winding. Pursuant to the principle of a current transformer, currents are thus generated in two secondary windings of the transformer and these currents reach the two transistors as base control currents. The windings are here polarized in such a manner that the voltages induced therein control the base of one transistor to be transmitting and the other to be blocked. The current flowing through the transistors is thus utilized to generate base currents of the desired polarity and amplitude for the control of the two transistors.
If the operating voltage applied to the series-connected transistors is derived from a three-phase network, e.g. a so-called three-phase current network with 380 V between the phases, it is possible to realize a sufficiently high and uniform operating voltage and a good performance for the entire circuit.
If, however, the operating voltage is generated from a 220 V single-phase network, unstable operation may result and, in particular, faulty base actuation of the two transistors. The reason for this is that, on the one hand, the power supply voltage is then less than it would be in a three-phase network and, on the other hand, the pulsating direct voltage of 100 Hz derived from the power supply periodically drops to almost zero due to less than ideal filtering. The circuit may then become unstable at higher operating frequencies than the resonant frequency which is given by the frequency determining members, particularly the capacitors, the inductance of a heating coil and the inductance of the transformer. Operating at frequencies higher than the resonant frequency is necessary, however, to be able to choke the power. In the prior art inverter, this power control is effected by changing the operating frequency.
Although it is possible to convert, by means of large filtering members, the pulsating direct voltage obtained from the power supply into an approximately uniform direct voltage having sufficiently low ripple, i.e. a voltage having an amplitude which does not drop to a noticeable degree, the cost of such filtering members is uneconomical, particularly if the amount of power to be processed is high.